refactor(quickadapter): deduplicate combined label-weight aggregate (#170) (#195)

refactor(quickadapter): deduplicate combined label-weight aggregate (#170) (#195)

Share the combined `select -> impute -> aggregate` pipeline between the value
path (`_compute_combined_label_weights`) and the dependency-mask path
(`compute_label_weight_imputation_dependency_mask`) via a single
`_compute_combined_label_weight_pipeline` helper returning a
`_CombinedWeightPipeline`. The mask branch now derives the per-component and
aggregate non-finite masks and the leading-stable release (#169) from the
shared result instead of re-running selection, imputation, and aggregation.

Behavior is bit-for-bit identical for `compute_label_weights` and
`compute_label_weight_imputation_dependency_mask` across all strategies,
aggregations, fill methods, and causal/non-causal modes, including raised
exception type, message, and order. The two imputers (`_impute_weights` and
`_causal_impute_weights`) stay distinct; the epsilon path keeps its causal
imputer through the shared helper.